When a modulated wave (sound wave) formed by a amplitude-modulating ultrasonic carrier wave at high sound pressure with an acoustic signal in an audible band is radiated in the air, because of nonlinearity of air, the sound speed becomes high at a location where the sound pressure is high and becomes low at a location where the sound pressure is low. Distortion, therefore, occurs in the waveform as the sound wave propagates in the air. It has been known that, as a result, the distortion is accumulated in the wave form and the carrier component is gradually attenuated as the sound wave propagates in the air, and the acoustic signal component in the audible band used for modulation is self-demodulated. This phenomenon is called a parametric array. Since the self-demodulated audible sound is carried by an ultrasonic wave and has sharp directionality, a speaker applying such a principle is called a parametric speaker, an ultra-directional speaker (ultrasonic speaker), or the like.
As a representative ultrasonic transducer that forms such an ultra-directional speaker (ultrasonic speaker), there are a piezoelectric ultrasonic transducer and an electrostatic ultrasonic transducer. The piezoelectric ultrasonic transducer is a resonant ultrasonic transducer that uses a piezoelectric element such as a piezoelectric material as a vibrator and drives it by utilizing a resonant frequency band thereof. Accordingly, the transducer is characterized in that high sound pressure can be efficiently generated, but the sound pressure-frequency characteristic is in a narrow band.
In contrast, the electrostatic ultrasonic transducer is an ultrasonic transducer that allows an electrostatic force to act between a fixed electrode and a thin electrode film to vibrate the electrode film. It is characterized in that the sound pressure-frequency characteristic is in a wide band.
Since the ultra-directional speaker (ultrasonic speaker) is required to generate high sound pressure, a resonant ultrasonic transducer is generally used in a conventional ultra-directional speaker. However, the conventional ultra-directional speaker is often evaluated as being lower in sound reproduction quality compared to a loudspeaker, and is only used for voice application such as a local announcement or an explanation of an exhibition. Thus, since the resonant ultrasonic transducer has sound pressure-frequency characteristics in a narrow band and limited drive frequencies, there are problems that the sound reproduction quality is difficult to improve and it is difficult to adjustment the reproduction range. Further, since the transducer is sensitive to excessive inputs and its elements are easy to break, there is another problem in that the transducer requires careful handling.
On the other hand, in the case of the electrostatic ultrasonic transducer, since the transducer has an output sound pressure per unit area that is lower than that of the resonant ultrasonic transducer, but sound pressure-frequency characteristics in a wide band, there are advantages that the improvement in reproduction quality is easily realized and the adjustment to the reproduction range is easy. Further, since the vibrator (film) is more flexible compared to that of the resonant ultrasonic transducer, there are advantages that the ultrasonic transducer is difficult to break with excessive inputs and there is no need to be so careful in handling as is the case of the resonant ultrasonic transducer.
Thus, it is more desirable that the ultra-directional speaker is formed using an electrostatic ultrasonic transducer in view of improvement in sound reproduction quality and easy handling.
Further, the electrostatic ultrasonic transducer is mainly divided into two types known as a pull-type and a push-pull type in structure thereof. The respective drawbacks and advantages are as follows.
FIG. 9 is a diagram for explaining the driving concept of a pull-type electrostatic ultrasonic transducer. An alternating current signal is superimposed on a direct current bias output from a DC bias supply and applied between a vibrating film (vibrating electrode film) 21 formed by depositing a conductive layer on a vibrating film (an insulating film or the like) and a fixed electrode 22. The vibrating film 21 is vibrated by the alternating current signal to output ultrasonic wave.
FIG. 9(a) shows an amplitude state of the vibrating film 21 in the case where a positive (+) side output of an alternating current signal is superimposed on the direct current bias and applied to the vibrating film 21, and FIG. 9(b) shows an amplitude state of the vibrating film 21 in the case where a negative (−) side output of an alternating current signal is superimposed on the direct current bias and applied to the vibrating film 21.
In the case of the state shown in FIG. 9(a), as the potential difference between the fixed electrode 22 and the vibrating film 21 becomes larger, a strong electrostatic force (attraction force) acts between the fixed electrode 22 and the vibrating film 21, and the central part of the vibrating film 21 is attracted toward the direction of the fixed electrode 22. In the case of the state shown in FIG. 9(b), as the potential difference between the fixed electrode 22 and the vibrating film 21 becomes smaller, an electrostatic force (attraction force) between the fixed electrode 22 and the vibrating film 21 becomes weaker, and the central part of the vibrating film 21 is drawn back toward the opposite direction to the fixed electrode 22 by a resilient restoration force. Thus, the vibrating film 21 vibrates according to the alternating current signal and outputs an ultrasonic wave.
In the pull-type electrostatic ultrasonic transducer, since there is no need to provide a through hole or the like for passing through a sound wave in the fixed electrode like a push-pull type electrostatic ultrasonic transducer (which will be described later), there are advantages that the aperture ratio is large and the sound pressure is easily secured. On the other hand, since the components that contribute to vibration are only the electrostatic attraction force and the resilient restoration force of the film, there is a drawback that the distortion in output waveform becomes larger.
Further, FIG. 10 is a diagram for explaining a driving concept of a push-pull type electrostatic ultrasonic transducer. In the push-pull type electrostatic ultrasonic transducer, a front-side fixed electrode 12 and a rear-side fixed electrode 13 are provided facing a vibrating film (vibrating electrode film) 11. A positive side DC bias is provided to the vibrating film 11 by a DC bias supply and an alternating current signal is applied between the front-side fixed electrode 12 and the rear-side fixed electrode 13.
FIG. 10(a) shows an amplitude state of the vibrating film 11 in the case where the alternating current signal is zero (0). The vibrating film 11 is located in a neutral position (in the middle of the front-side fixed electrode 12 and the rear-side fixed electrode 13). FIG. 10(b) shows an amplitude state of the vibrating film 11 in the case where the negative voltage of the alternating current signal is applied to the front-side fixed electrode 12 and the positive voltage of the alternating current signal is applied to the rear-side fixed electrode 13. The central part of the vibrating film 11 is attracted toward the direction of the front-side fixed electrode 12 by an electrostatic force (repulsion force) between the rear-side fixed electrode 13 and itself and an electrostatic force (attraction force) between the front-side fixed electrode 12 and itself.
FIG. 10(c) shows an amplitude state of the vibrating film 11 in the case where the positive voltage of the alternating current signal is applied to the front-side fixed electrode 12 and the negative voltage of the alternating current signal is applied to the rear-side fixed electrode 13. The central part of the vibrating film 11 is attracted toward the direction of the rear-side fixed electrode 13 by an electrostatic force (repulsion force) between the front-side fixed electrode 12 and itself and an electrostatic force (attraction force) between the rear-side fixed electrode 13 and itself.
Thus, the vibrating film 11 vibrates according to the alternating current signal and outputs sound waves.
In the push-pull type electrostatic ultrasonic transducer, since both the electrostatic attraction force and the electrostatic repulsion force act on the vibrating film, that is, the electrostatic forces symmetrically act positively and negatively, there is an advantage that the distortion in output waveforms become smaller. On the other hand, since the sound wave is output through the through hole provided in the fixed electrode, there are drawbacks that the aperture ratio is smaller and the sound pressure is difficult to secure.
By the way, in the case of using an electrostatic ultrasonic transducer for the ultra-directional speaker, there is a specific problem that, even when ideal amplitude-modulated waves in an ultrasonic wave band are input to the speaker, if the positively and negatively asymmetric distortion of the waveforms (carrier wave) output from the ultrasonic transducer are large, the distortion component becomes an audible sound component, audible sound is directly output from the speaker other than the ultrasonic wave component, and the directionality of auditory sense becomes low. This is because the electrostatic ultrasonic transducer has a sound pressure characteristic in a wide frequency band (when the audible sound itself is directly input, some degree of sound pressure is provided), and a problem specific to the ultrasonic transducer having wide band characteristics. Accordingly, in order to avoid the above described problems, it is more desirable to use a push-pull type having smaller distortion in output waveform than a pull-type.
In the case where an ultra-directional speaker (ultrasonic speaker) is formed by a push-pull type ultrasonic transducer, since through holes for passing through sound waves are provided in both upper and lower fixed electrodes that sandwich the vibrating film in the conventional ultrasonic transducer, the sound wave is emitted toward both the front surface and the rear surface (e.g., see Patent Document 1).
A case where such an ultra-directional speaker is mounted on equipment such as a projector, for example, and screen sound is realized by reflecting sound waves on a screen for projecting a video will be considered. In this case, when the speaker is provided so as to overhang to the outside of the housing of the projector, there is a problem that realistic sensation is hindered because a person watching the screen from the rear side of the projector directly hears not only the sound reflected by the screen but also the sound from the speaker of the projector main body. On the other hand, there is a problem that realistic sensation is also hindered because the sound wave radiated from the speaker rear surface is reflected on the rear wall and a person watching the screen in front of the projector hears not only the sound reflected by the screen but also the same sound from the rear side.
Further, when the speaker is provided inside the housing of the projector, the above described problem does not occur because the sound wave radiated from the rear surface is blocked by the housing or internal structure and the sound wave is radiated only toward the front side. However, the sound wave reflected at a point-blank range of the housing or internal structure directly bounces back to the vibrating film of the ultrasonic transducer and disturbs the vibration of the vibrating film. As a result, there is a problem that the directionality and sound quality of sound wave output from the front surface becomes deteriorated.
[Patent Document No. JP-A-6-209499]